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HARDY INEQUALITIES IN STRIPS ON RULED SURFACES
DAVID KREJ
ˇ
CI
ˇ
R
´
IK
Received 17 August 2005; Accepted 8 November 2005
We consider the Dirichlet Laplacian in infinite two-dimensional strips defined as uniform
tubular neighbourhoods of curves on ruled surfaces. We show that the negative Gauss
curvature of the ambient surface gives rise to a Hardy inequality and we use this to prove
certain stability of spectrum in the case of asymptotically straight strips about mildly
pertur bed geodesics.
Copyright © 2006 David Krej
ˇ
ci
ˇ
r
´
ık. This is an open access article distributed under the
Creative Commons Attribution License, which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is properly cited.
1. Introduction
Problems linking the geometry of two-dimensional manifolds and the spectrum of as-
sociated Laplacians have been considered for more than a century. While classical mo-
tivations come from theories of elasticity and electromagnetism, the same rather simple
models can be also remarkably successful in describing even rather complicated phenom-
ena in quantum heterostructures. Here, an enormous amount of recent research has been
undertaken on both the theoretical and experimental aspects of binding in curved strip-
like waveguide systems.

More specifically, as a result of theoretical studies, it is well known now that the Dir ich-
let Laplacian in an infinite planar strip of uniform width always possesses eigenvalues
below its essential spectrum whenever the strip is curved and asymptotically straight.
We refer to [13, 15] for initial proofs and to [8, 19, 21]forreviewswithmanyref-
erences on the topic. The existence of the curvature-induced bound states is interest-
ing from several respects. First of all, one deals with a purely quantum effect of geo-
metrical origin, with negative consequences for the electronic transport in nanostruc-
tures. From the mathematical point of view, the strips represent a class of noncom-
pact noncomplete manifolds for which the spectral results of this type are nontriv ial,
too.
Hindawi Publishing Corporation
Journal of Inequalities and Applications
Volume 2006, Article ID 46409, Pages 1–10
DOI 10.1155/JIA/2006/46409
2 Hardy inequalities in strips on ruled surfaces
At the same time, a couple of results showing that the attractive interaction due to
bending can be eliminated by appropriate additional perturbations have been established
quite recently. Dittrich and K
ˇ
r
´
ı
ˇ
z[7] demonst rated that the discrete spectrum of the Lapla-
cian in any asymptotically straight planar st rip is empty provided the curvature of the
boundary curves does not change sign and the Dirichlet condition on the locally shorter
boundary is replaced by the Neumann one. A different proof of this result and an exten-
sion to Robin boundary conditions were performed in [14]. Ekholm and Kova
ˇ
r

´
ık [10]ob-
tained the same conclusion for the purely Dir ichlet Laplacian in a mildly curved strip by
introducing a local mag netic field perpendicular to the strip. The purpose of the present
paper is to show that the same types of repulsive interaction can be created if the ambient
space of the strip is a negatively curved manifold instead of the Euclidean plane.
A spectral analysis of the Dirichlet Laplacian in infinite strips embedded in curved
two-dimensional manifolds was performed for the first time by the present author in
[18]. He derived a sufficient condition which guarantees the existence of discrete eigen-
values in asymptotically straight strips; in particular, the bound states exist in strips on
positively curved surfaces and in curved strips on flat surfaces. He also performed heuris-
tic considerations suggesting that the discrete spectrum might be empty for certain strips
on negatively curved surfaces. Similar conjectures were also made previously for strips on
ruled surfaces in [5]. However, a rigorous treatment of the problem remained open.
In the present paper, we derive several Hardy inequalities for mildly curved strips on
ruled surfaces, which proves the conjecture for this class of strips. A ruled surface is gen-
erated by straight lines translating along a curve in the Euclidean space; hence its Gauss
curvature is always nonpositive. The reason why we restrict to ruled surfaces in this paper
is due to the fact that the Jacobi equation determining the metric in geodesic coordinates
is explicitly solvable, so that rather simple formulae are available. Nevertheless, it should
be possible to extend the present ideas to other classes of nonpositively curved surfaces
for which more precise information a bout geodesics are available.
Hardy inequalities represent a powerful technical tool in more advanced theoretical
studies of elliptic operators. We refer to the book [22] for an exhaustive study and gen-
eralizations of the original inequality due to Hardy. Interesting Hardy inequalities on
noncompact Riemannian manifolds were established in [2]. In the quantum-waveguide
context, vari ous types of Hardy inequality were derived in [1, 10, 11]inordertoprove
certain stability of spectrum of the Laplacian in tubular domains.
Here the last reference is the closest to the issue of the present paper. Indeed, the au-
thors of [11] considered a three-dimensional tube constructed by translating a noncircu-

lar two-dimensional cross-section along an infinite curve and obtained that the twisting
due to an appropriate construction eliminates the curvature-induced discrete spectrum
in the regime of mild curvature. Formally, the strips of the present paper can be viewed
as a singular case of [11] when the cross-section is replaced by a segment and the effect
of twisting is hidden in the curvature of the ambient space. While [11] and the present
paper exhibit these similarity features, and also the technical handling of the problems is
similar, they differ in some respects. On the one hand, the present situation is simpler,
since it happens that the negative curvature of the ambient space gives rise to an explicit
repulsive potential (cf. (3.6)) which leads to a Hardy inequality in a more direct way than
David Krej
ˇ
ci
ˇ
r
´
ık 3
in [11]. On the other hand, we do not perform the unitary transformation of [11]in
ordertoreplacetheLaplacianontheHilbertspaceofacurvedstripbyaSchr
¨
odinger-
type operator on a “straighten” Hilbert space, but we work directly with “curved” Hilbert
spaces. T his technically more complicated approach has an advantage that we need to
impose no conditions whatsoever on the derivatives of curvatures.
Although we are not aware of a direct physical interpretation of the Laplacian in infi-
nite strips if the ambient space has a nontrivial curvature, there exists an indirect motiva-
tion coming from the theory of quantum layers studied in [3, 9, 20]. In these references,
the Dirichlet Laplacian in tubular neighbourhoods of a surface in the Euclidean space is
used for the quantum Hamiltonian (cf. [12] for a similar model). Taking our str ip as the
reference surface, the layer model of course differs from the present one, but a detailed
study of the latter is impor t ant to understand certain spectral properties of the former.

Similar layer problems are also considered in other areas of physics away from quantum
theories, (cf. [16]). Finally, the present problem is a mathematically interesting one in the
context of spectral geometry.
The organization of the paper is as follows. The ambient ruled surface, the strip,
and the corresponding Dirichlet Laplacian are properly defined in the preliminaries in
Section 2.InSection 3, we consider the special situation of the strip being straight in
a generalized sense. If the Gauss curvature of such a strip does not vanish identically
and the st rip is thin enough, we derive a central Hardy inequality of the present paper,
(cf. Theorem 3.1). In fact, the latter is established by means of a “local” Hardy inequal-
ity, (cf. (3.7)), which might be also interesting for applications. In Section 4,weapply
Theorem 3.1 to mildly curved strips and prove certain stability of spectrum, (cf. Theorem
4.1). As an intermediate result, we obtain a general Hardy inequality for mildly curved
strips on ruled surfaces (cf. (4.7)).
2. Preliminaries
Given two bounded continuous functions κ and τ defined on
R with κ being positive, let
Γ :
R →R
3
be the unit-speed curve whose curvature and torsion are κ and τ, respectively.
Γ is determined uniquely up to congruent transformations and possesses a distinguished
C
1
-smooth Frenet frame {
˙
Γ,N,B
} consisting of tangent, normal, and binormal vector
fields, respectively (cf. [17, Chapter 1]). It is also convenient to include the case of κ and
τ being equal to zero identically, which corresponds to Γ being a straight line with a
constant Frenet frame.

Given a bounded C
1
-smooth function θ defined on R, let us introduce the mapping
ᏸ :
R
2
→ R
3
via
ᏸ(s,t):
= Γ(s)+t

N(s)cosθ(s) −B(s) sinθ(s)

. (2.1)
ᏸ represents a ruled surface (cf. [17, Definition 3.7.4]) provided it is an immersion. The
latter is ensured by requiring that the metric tensor G
≡ (G
ij
)inducedbyᏸ, that is,
G
ij
:=

∂
i
ᏸ

·


∂
j
ᏸ

, i, j ∈{1,2}, (2.2)
4 Hardy inequalities in strips on ruled surfaces
where the dot denotes the scalar product in
R
3
, to be positive definite. Employing the
Serret-Frenet formulae (cf. [17, Section 1.3]), we find
G
=

h
2
0
01

, h(s,t):=


1 −tκ(s)cosθ(s)

2
+ t
2

τ(s) −
˙

θ(s)

2
. (2.3)
Hence, it is enough to assume that t is sufficiently small so that the first term in the square
root defining h never vanishes.
More restrictively, given a positive number a, we always assume that
a
κcosθ
∞
< 1, (2.4)
so that also h
−1
is bounded, and define a ruled strip of width 2a to be the Riemannian
manifold
Ω :
=

R ×
(−a,a),G

. (2.5)
That is, Ω is a noncompact and noncomplete surface which is fully characterized by the
functions κ, τ, θ and the number a. It is easy to verify that the Gauss curvature K of Ω is
nonpositive, namely,
K
=−

τ −
˙

θ

2
h
−4
. (2.6)
Moreover, if the mapping ᏸ is injective, then the image ᏸ(
R ×(−a,a)) has indeed the ge-
ometrical meaning of a non-self-intersecting strip and Ω represents its parameterization
in geodesic coordinates.
Remark 2.1. In (2.3), let us write k instead of κcos θ and σ instead of τ
−
˙
θ,andassume
that k and σ are given bounded continuous functions on
R. Then, abandoning the geo-
metrical interpretation in terms of ruled surfaces based on Γ,(2.5) can be considered as
an abstract Riemannian manifold, with a
k
∞
< 1 being the only restriction. The spec-
tral results of this paper extend automatically to this more general situation by applying
the above identification.
Our object of interest is the Dirichlet Laplacian in Ω, that is, the unique selfadjoint
operator
−Δ
Ω
D
associated with the closure of the quadratic form Q defined in the Hilbert
space

Ᏼ :
= L
2
(Ω) ≡L
2

R ×
(−a,a),h(s, t)dsdt

(2.7)
by the prescription
Q[ψ]:
=

∂
i
ψ, G
ij
∂
j
ψ

Ᏼ
, ψ ∈D(Q):=C
∞
0

R ×
(−a,a)


, (2.8)
where (G
ij
):= G
−1
and the summation is assumed over the indices i, j ∈{1,2}.Given
ψ
∈ D(Q), we have
Q[ψ]
=


h
−1
∂
1
ψ


2
Ᏼ
+


∂
2
ψ


2

Ᏼ
. (2.9)
David Krej
ˇ
ci
ˇ
r
´
ık 5
Under the stated assumptions, it is clear that the form domain of
−Δ
Ω
D
is just the Sobolev
space W
1,2
0
(R ×(−a,a)). If ᏸ is injective, then −Δ
Ω
D
is nothing else than the Dirichlet
Laplaciandefinedintheopensubsetᏸ(
R × (−a,a)) of the ruled surface (2.1)andex-
pressed in the “coordinates” (s,t).
3. Geodesic strips
The r uled strip Ω is called a geodesic strip and is denoted by Ω
0
if the reference curve Γ is
a geodesic on ᏸ.Sinceκcosθ is the geodesic curv ature of Γ (when the latter is considered
as a curve on ᏸ), it is clear that Ω is a geodesic strip provided that Γ is either a straight

line (i.e., geodesic in
R
3
) or the straight lines t →ᏸ(s,t) −Γ(s) generating the ruled sur-
face (2.1) are tangential to the binormal vector field for each fixed s. The metric (2.3)
corresponding to Ω
0
acquires the form
G
0
:=

h
2
0
0
01

, h
0
(s,t):=

1+t
2

τ(s) −
˙
θ(s)

2

, (3.1)
and we denote by Ᏼ
0
, Q
0
,and−Δ
Ω
0
D
, respectively, the corresponding Hilbert space defined
in analogy to (2.7), the corresponding quadr atic form defined in analogy to (2.8), and the
associated Dirichlet Laplacian in Ω
0
.
If τ
−
˙
θ is equal to zero identically, that is, Ω
0
is a flat surface due to (2.6), it is easy to
see that the spectrum of
−Δ
Ω
0
D
coincides with the interval [E
1
,∞), where
E
1

:=
π
2
(2a)
2
(3.2)
is the lowest eigenvalue of the Dirichlet Laplacian in (
−a,a). In this section, we prove that
the presence of a Gauss curvature leads to a Hardy inequality for the difference
−Δ
Ω
0
D
−E
1
,
which has important consequences for the stability of spectrum.
Theorem 3.1. Given a positive number a and bounded continuous functions τ and
˙
θ,letΩ
0
be the Riemannian manifold (R ×(−a,a),G
0
) with the metric given by (3.1). Assume that
τ
−
˙
θ is not identically zero and that a
τ −
˙

θ

∞
<
√
2.Then,forallψ ∈W
1,2
0
(R ×(−a,a))
and any s
0
such that (τ −
˙
θ)(s
0
) =0,
Q
0
[ψ] −E
1
ψ
2
Ᏼ
0
≥ c


ρ
−1
ψ



2
Ᏼ
0
with ρ(s,t):=

1+(s −s
0
)
2
. (3.3)
Here c is a positive constant which depends on s
0
, a,andτ −
˙
θ.
It is possible to find an explicit lower bound for the constant c;wegiveanestimatein
(3.15)below.
Theorem 3.1 implies that the presence of a Gauss curvature represents a repulsive in-
teraction in the sense that there is no spectrum below E
1
for all small potential-type per-
turbations having ᏻ(s
−2
) decay at infinity. Moreover, in Section 4, we show that this is
also the case for appropriate perturbations of the metric (3.1).
6 Hardy inequalities in strips on ruled surfaces
In order to prove Theorem 3.1, we introduce the function λ :
R →R by

λ(s):
= inf
ϕ∈C
∞
0
((−a,a))\{0}

a
−a


˙
ϕ(t)


2
h
0
(s,t)dt

a
−a


ϕ(t)


2
h
0

(s,t)dt
−E
1
(3.4)
and keep the same notation for the function λ
⊗1onR ×(−a,a). We have the following
lemma.
Lemma 3.2. Under the hypotheses of Theorem 3.1 , λ is a continuous nonnegative function
which is not identically equal to zero.
Proof. For any fix s
∈ R, we make the change of test function φ :=

h
0
(s,·)ϕ,integrateby
parts, and arrive at
λ(s)
= inf
φ∈C
∞
0
((−a,a))\{0}

a
−a



˙
φ(t)



2
−E
1


φ(t)


2
+ V(s,t)


φ(t)


2

dt

a
−a


φ(t)


2
dt

(3.5)
with
V(s,t):
=

τ(s) −
˙
θ(s)

2

2 −t
2

τ(s) −
˙
θ(s)

2

4h
0
(s,t)
4
. (3.6)
Under the hypotheses of Theorem 3.1, the function V is clearly continuous, nonnega-
tive, and not identically zero. These facts together with the Poincar
´
e inequality


a
−a
|
˙
φ
|
2
≥
E
1

a
−a
|φ|
2
valid for any φ ∈C
∞
0
((−a,a)) yield the claims of the lemma. 
Assuming that the conclusion of Lemma 3.2 holds and using t he definition (3.4), we
get the estimate
Q
0
[ψ] −E
1
ψ
2
Ᏼ
0
≥



h
−1
0
∂
1
ψ


2
Ᏼ
0
+


λ
1/2
ψ


2
Ᏼ
0
(3.7)
valid for any ψ
∈ C
∞
0
(R ×(−a,a)). Neglecting the first term on the right-hand side of

(3.7), the inequality is already a Hardy inequality. However, for applications, it is more
convenient to replace the Hardy weight λ in (3.7) by the positive function cρ
−2
of Theorem
3.1. T his is possible by employing the contribution of the first term based on the following
lemma.
Lemma 3.3. For any ψ
∈ C
∞
0
(R ×(−a,a)),

1+a
2
τ−
˙
θ

2
∞

−1/2


ρ
−1
ψ


2

Ᏼ
0
≤ 16

1+a
2
τ−
˙
θ

2
∞

1/2


h
−1
0
∂
1
ψ


2
Ᏼ
0
+

2+

64
|I|
2



χ
I
ψ


2
Ᏼ
0
,
(3.8)
where I is any bounded subinterval of
R, χ
I
denotes the characteristic function of the set
I
×(−a,a),andρ is the function of Theorem 3.1 with s
0
being the centre of I.
David Krej
ˇ
ci
ˇ
r
´

ık 7
Proof. The lemma is based on the following version of the one-dimensional Hardy in-
equality:

R


u(x)


2
x
2
dx ≤4

R


˙
u(x)


2
dx (3.9)
valid for all u
∈ W
1,2
(R)withu(0) = 0. Put b :=|I|/2. We define the function f : R →R
by
f (s):

=
⎧
⎪
⎪
⎨
⎪
⎪
⎩
1for


s −s
0


≥
b,


s −s
0


b
for


s −s
0



<b,
(3.10)
and keep the same notation for the function f
⊗1onR ×(−a,a). For any ψ ∈C
∞
0
(R ×
(−a,a)), let us write ψ = fψ+(1− f )ψ.Applying(3.9) to the function s → ( fψ)(s,t)
with t fixed, we arrive at

|ψ|
2
ρ
2
≤ 2

|fψ|
2
ρ
2
−1
+2

χ
I


(1 − f )ψ



2
≤ 16



∂
1
f


2
|ψ|
2
+16

|
f |
2


∂
1
ψ


2
+2

χ

I


(1 − f )ψ


2
≤ 16



∂
1
ψ


2
+

2+
16
b
2


χ
I
|ψ|
2
,

(3.11)
where the integration sign indicates the integration over
R ×(−a,a). Recalling the defi-
nition of Ᏼ
0
and using the estimates
1
≤ h
2
0
≤ 1+a
2
τ −
˙
θ

2
∞
, (3.12)
the lemma follows at once.

Now we are in a position to prove Theorem 3.1.
Proof of Theorem 3.1. It suffices to prove the theorem for functions ψ from the dense sub-
space C
∞
0
(R ×(−a,a)). Assume the hypotheses of Theorem 3.1 so that the conclusion of
Lemma 3.2 holds. Let I be any closed interval on which λ is positive. Writing



λ
1/2
ψ


2
Ᏼ
0
= 


λ
1/2
ψ


2
Ᏼ
0
+(1−

)


λ
1/2
ψ


2

Ᏼ
0
with 
∈
(0,1], (3.13)
neglecting the second term of this decomposition, estimating the first one by an integral
over I
×(−a,a), and applying Lemma 3.3, the inequality (3.7)yields
Q
0
[ψ] −E
1
ψ
2
Ᏼ
0
≥

1 −16 min
I
λ

2+
64
|I|
2

−1

1+a

2
τ −
˙
θ

2
∞

1/2



h
−1
0
∂
1
ψ


2
Ᏼ
0
+  min
I
λ

2+
64
|I|

2

−1

1+a
2
τ −
˙
θ

2
∞

−1/2


ρ
−1
ψ


2
Ᏼ
0
.
(3.14)
8 Hardy inequalities in strips on ruled surfaces
Choosing
 as the minimum between 1 and the value such that the first term on the
right-hand side of the last estimate vanishes, we get the claim of Theorem 3.1 with

c
≥ min

min
I
λ

2+64/|I|
2

1+a
2
τ −
˙
θ

2
∞

1/2
,
1
16

1+a
2
τ −
˙
θ


2
∞


. (3.15)

4. Mildly curved strips
Recall that the spectrum of
−Δ
Ω
0
D
coincides with the interval [E
1
,∞) provided that the
Gauss curvature (2.6) vanishes everywhere in the geodesic strip Ω
0
. On the other hand,
it was proved in [18]that
−Δ
Ω
D
always possesses a spectrum below E
1
provided that the
Gauss curvature (2.6) vanishes everywhere but Γ is not a geodesic on ᏸ. In this section,
we use the Hardy inequality of Theorem 3.1 to show that the presence of Gauss curvature
prevents the spectrum to descend even if Γ is mildly curved.
Theorem 4.1. Given a positive number a and bounded continuous functions κ, τ,and
˙

θ,
let Ω be the Riemannian manifold (2.5) with the metric given by (2.3). Assume that τ
−
˙
θ is
not identically zero and that a
τ −
˙
θ

∞
<
√
2. Assume also that for all s ∈ R,


κ(s)cosθ(s)


≤
ε(s):=
ε
0
1+s
2
with ε
0
∈

0,a

−1

. (4.1)
Then there exists a positive number C such that ε
0
≤ C implies that
−Δ
Ω
D
≥ E
1
. (4.2)
Here C depends on a and on the constants c and s
0
of Theorem 3.1.
As usual, the inequalit y (4.2) is to be considered in the sense of forms. Actually, a
stronger, Hardy-type inequality holds true, (cf. (4.7)).
An explicit lower bound for the constant C is given by the estimates made in the proof
of Theorem 4.1.
As a direct consequence of Theorem 4.1, we get that the spectrum [E
1
,∞)isstableas
a set provided that the difference τ
−
˙
θ vanishes at infinity.
Corollary 4.2. In addition to hypotheses of Theorem 4.1, assume that τ(s)
−
˙
θ(s) tends to

zero as
|s|→∞. Then
spec

−
Δ
Ω
D

=

E
1
,∞

. (4.3)
Proof. Following the proof of [4, Section 3.1] or [19, Section 5] based on a general charac-
terization of essential spectrum adopted from [6], it is possible to show that the essential
spectrum
−Δ
Ω
D
coincides with the interval [E
1
,∞), while Theorem 4.1 ensures that there
is no spectrum below E
1
. 
Proof of Theorem 4.1. Let ψ belong to C
∞

0
(R ×(−a,a)). The proof is based on an algebraic
comparison of Q[ψ]
−E
1
ψ
2
Ᏼ
with Q
0
[ψ] −E
1
ψ
2
Ᏼ
0
and the usage of Theorem 3.1.For
David Krej
ˇ
ci
ˇ
r
´
ık 9
every (s,t)
∈ R ×(−a,a), we have
f
−
(s):=





1 −
aε(s)

2+aε(s)

1+a
2
τ −
˙
θ

2
∞
≤
h(s,t)
h
0
(s,t)
≤

1+aε(s)

2+aε(s)

=
: f
+

(s). (4.4)
Here the lower bound is well defined and positive provided that ε
0
≤ (3a)
−1
, and both
bounds behave as 1 + ᏻ(ε(s)) as ε
0
→ 0; we keep the same notation f
±
for the functions
f
±
⊗1onR ×(−a,a). Consequently,
Q[ψ]
−E
1
ψ
2
Ᏼ
≥

R×(−a,a)
f
−1
+
h
−1
0



∂
1
ψ


2
+

R
ds f
−
(s)

a
−a
dth
0
(s,t)



∂
2
ψ(s, t)


2
−E
1



ψ(s, t)


2

−
E
1

R×(−a,a)

f
+
− f
−

h
0
|ψ|
2
.
(4.5)
Sincetheterminthesecondlineisnonnegativedueto(3.4)andLemma 3.3,wecan
further estimate as foll ows:
Q[ψ]
−E
1
ψ

2
Ᏼ
≥ min

f
+
(0)
−1
, f
−
(0)

Q
0
[ψ]−E
1
ψ
2
Ᏼ
0

−
E
1

R×(−a,a)

f
+
− f

−

h
0
|ψ|
2
.
(4.6)
Using Theorem 3.1,wefinallyobtain
Q[ψ]
−E
1
ψ
2
Ᏼ
≥


w
1/2
ψ


2
Ᏼ
0
, (4.7)
where
w(s,t):
=

c min

f
+
(0)
−1
, f
−
(0)

1+

s −s
0

2
−E
1

f
+
(s) − f
−
(s)

(4.8)
is positive for all sufficiently small ε
0
. 
Acknowledgments

This work has been supported by the Czech Academy of Sciences and its Grant Agency
within the Projects IRP AV0Z10480505 and A100480501.
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David Krej
ˇ
ci
ˇ
r
´
ık: Department of Theoretical Physics, Nuclear Physics Institute,
Academy of Sciences of the Czech Republic, 250 68
ˇ
Re
ˇ
z, Czech Republic

E-mail address: